Positioning device, positioning control method, positioning control program, and computer-readable recording medium having positioning control program recorded thereon

ABSTRACT

A positioning device comprising: a peak frequency determination section which determines a peak frequency which is a reception frequency corresponding to a maximum correlation value of a specific positioning base code replica and a positioning base code carried on a radio wave from a specific transmission source; a reference frequency calculation section which calculates a low frequency which is a frequency lower than the peak frequency and a high frequency which is a frequency higher than the peak frequency; a reference correlation value calculation section which calculates the correlation value corresponding to the low frequency and the correlation value corresponding to the high frequency; a corrected peak frequency calculation section which calculates a corrected peak frequency based on the correlation value corresponding to the peak frequency, the peak frequency, the correlation value corresponding to the low frequency, the low frequency, the correlation value corresponding to the high frequency, and the high frequency; and a radio wave reception section which receives the radio wave using the corrected peak frequency.

Japanese Patent Application No. 2006-81532 filed on Mar. 23, 2006 andJapanese Patent Application No. 2006-81533 filed on Mar. 23, 2006, arehereby incorporated by reference in their entirety.

BACKGROUND OF THE INVENTION

The present invention relates to a positioning device, a positioningcontrol method, a positioning control program, and a computer-readablerecording medium having a positioning control program recorded thereon.

A positioning system has been used which locates the present position ofa GPS receiver utilizing a satellite navigation system such as a globalpositioning system (GPS).

The GPS receiver receives a clear and acquisition or coarse and access(C/A) code which is one type of pseudo-random noise code (hereinaftercalled “PN code”) carried on a radio wave from a GPS satellite(hereinafter called “satellite radio wave”) based on a navigationmessage indicating the orbit of the GPS satellite and the like(including approximate satellite orbital information: almanac, precisesatellite orbital information: ephemeris, and the like). The C/A code isa code which forms the basis for positioning.

The GPS receiver determines the GPS satellite which has transmitted thereceived C/A code, and calculates the distance (pseudo-range) betweenthe GPS satellite and the GPS receiver based on the transmission timeand the reception time of the C/A code, for example. The GPS receiverlocates the position of the GPS receiver based on the pseudo-rangebetween the GPS receiver and each of three or more GPS satellites andthe position of each GPS satellite in the satellite orbit (seeJP-A-10-339772).

The GPS receiver synchronizes the received C/A code with a C/A codereplica in the GPS receiver, and calculates a phase indicating themaximum correlation value (hereinafter called “code phase”). The GPSreceiver calculates the pseudo-range using the code phase.

Specifically, the C/A code has a bit rate of 1.023 Mbps and a codelength of 1023 chips. Therefore, it is considered that the C/A codesline up in units of about 300 kilometers (km) at which a radio waveadvances in 1 millisecond (ms). Therefore, the pseudo-range can becalculated by calculating the number of C/A codes existing between theGPS satellite and the GPS receiver from the position of the GPSsatellite in the satellite orbit and the approximate position of the GPSreceiver, and determining the phase of the C/A code.

Since the C/A code is carried on the satellite radio wave, it isnecessary to synchronize the C/A codes and synchronize the carrierfrequency (intermediate-frequency (IF) carrier frequency) of thereceived satellite radio wave with the frequency inside the GPS receiver(hereinafter called “frequency synchronization”) in order to accuratelysynchronize the C/A codes.

When the correlation result (coherent result) can be output in timeunits as short as 1 millisecond (ms) due to the high signal strength ofthe satellite radio wave, the frequencies can be synchronized by forminga phase locked loop (PLL) which corrects the frequency based on thecoherent result (see paragraph [0020] of JP-A-2003-98244, for example).

However, the frequencies cannot be synchronized using the PLL when thestrength of the satellite radio wave is low. It becomes impossible tosynchronize the codes with the lapse of time.

A technology has been proposed which sets an estimated IF carrierfrequency by estimating the original IF carrier frequency, and reducesthe difference in signal level between a frequency higher than theestimated IF carrier frequency by a specific value and a frequency lowerthan the estimated IF carrier frequency by the specific value to bringthe estimated IF carrier frequency closer to the true IF carrierfrequency (see JP-A-2003-255036, for example).

In order to cause the phase of the C/A code replica generated in the GPSreceiver to coincide with the phase of the received C/A code, acorrelation process is performed while changing the phase of the C/Acode replica. Note that the correlation process is performed whilechanging the reception frequency. Description thereof is omitted fromthis specification.

A graph indicating the correlation value in coordinates of which thehorizontal axis indicates the phase and the vertical axis indicates thecorrelation value theoretically forms an isosceles triangle having themaximum correlation value as the vertex. A method has been used whichgenerates C/A code replicas with a phase (EARLY) or a phase (LATE) whichadvances or is delayed by a specific amount from a phase (PUNCTUAL)considered to be an intermediate phase, correlates the C/A code replicaswith the phases EARLY and LATE with the received C/A code, and controlsthe phases of the C/A code replicas so that the correlation valuesbecome equal. The intermediate phase between the phases EARLY and LATEwhen the correlation values of the phases EARLY and LATE are equal isestimated to be the phase of the received C/A code.

The signal from the GPS satellite may reach the GPS receiver as anindirect wave which enters after being reflected by a building or thelike (hereinafter called “multipath”) in addition to a direct wave. Inthis case, the isosceles triangle having the maximum correlation valueas the vertex is deformed, whereby the phase of the received C/A codecannot be accurately estimated by the above method.

A technology has been proposed in which the correlation process isperformed while reducing the difference between the phases EARLY andLATE (narrow correlator technology) (JP-A-2000-312163, for example).

However, the following two problems occur when the signal strength ofthe satellite radio wave is extremely low.

A first problem is that it is necessary to appropriately determine theestimated IF carrier frequency. Specifically, the estimated IF carrierfrequency cannot be appropriately determined when the signal strength ofthe satellite radio wave is extremely low.

A second is that, when the signal strength of the satellite radio waveis extremely low, the correlation values of the phases EARLY and LATEbecome equal at a plurality of positions in the graph indicating thecorrelation value, as shown in FIG. 26. For example, when the phaseEARLY is a phase Qe1 and the phase LATE is a phase Qe2, the correlationvalues of the phases Qe1 and Qe2 are equal, and the intermediate phasebetween the phases Qe1 and Qe2 is a phase Qe3. However, the phase Qe3differs from the true phase Qr.

As described above, when the signal strength is extremely low (electricfield is weak), the phase of the received C/A code may not be accuratelyestimated by the above narrow correlator technology. In thisspecification, the term “signal strength” is used synonymously with theterm “radio wave strength”.

SUMMARY

According to one aspect of the invention, there is provided apositioning device comprising:

a peak frequency determination section which determines a peak frequencywhich is a reception frequency corresponding to a maximum correlationvalue of a specific positioning base code replica and a positioning basecode carried on a radio wave from a specific transmission source;

a reference frequency calculation section which calculates a lowfrequency which is a frequency lower than the peak frequency and a highfrequency which is a frequency higher than the peak frequency;

a reference correlation value calculation section which calculates thecorrelation value corresponding to the low frequency and the correlationvalue corresponding to the high frequency;

a corrected peak frequency calculation section which calculates acorrected peak frequency based on the correlation value corresponding tothe peak frequency, the peak frequency, the correlation valuecorresponding to the low frequency, the low frequency, the correlationvalue corresponding to the high frequency, and the high frequency; and

a radio wave reception section which receives the radio wave using thecorrected peak frequency.

According to another aspect of the invention, there is provided apositioning device comprising:

a first correlation value calculation section which performs acorrelation process of a specific positioning base code replica and apositioning base code and calculates a correlation value in units offirst sampling phases which are phases at intervals of a first dividedphase width which is a phase width obtained by equally dividing a phaserange specified by a base unit of a positioning base code formed of aplurality of base units from a transmission source into at least threesections;

a first phase determination section which determines a first phase whichis a sampling phase corresponding to the maximum correlation value;

a first positioning phase calculation section which calculates a firstpositioning phase used for positioning based on three consecutive firstsampling phases including the first phase and the correlation valuescorresponding to the three consecutive first sampling phases includingthe first phase; and

a first located position calculation section which calculates a presentlocated position based on the first positioning phases corresponding tothree or more of the transmission sources.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

FIG. 1 is a schematic view showing a terminal and the like according toa first embodiment.

FIG. 2 is a schematic view showing the main hardware configuration ofthe terminal according to the first embodiment.

FIG. 3 is a schematic view showing the configuration of a GPS deviceaccording to the first embodiment.

FIG. 4 is a schematic view showing the main software configuration ofthe terminal according to the first embodiment.

FIG. 5 is a view illustrative of a first estimated frequency calculationprogram according to the first embodiment.

FIG. 6A is a view illustrative of the process of a first correlationprogram according to the first embodiment, FIG. 6B is a viewillustrative of a correlation process, and FIG. 6C is a viewillustrative of the relationship between a phase and a correlationvalue.

FIG. 7 is a schematic view showing a positioning method according to thefirst embodiment.

FIG. 8 is a view illustrative of a peak frequency determination programaccording to the first embodiment.

FIG. 9A is a view showing the relationship between a frequency and acorrelation value which is illustrative of a second estimated frequencycalculation program according to the first embodiment, and FIG. 9B is aview illustrative of the process of a second estimated frequencycalculation program.

FIG. 10A is a view showing the relationship between a frequency and acorrelation value which is illustrative of a second estimated frequencycalculation program according to the first embodiment, and FIG. 10B is aview illustrative of the process of the second estimated frequencycalculation program.

FIG. 11 is a view illustrative of a second phase determination programaccording to the first embodiment.

FIG. 12 is a schematic flowchart showing an operation example of theterminal according to the first embodiment.

FIG. 13 is a schematic flowchart showing an operation example of theterminal according to the first embodiment.

FIG. 14 is a schematic view showing a terminal and the like according toa second embodiment.

FIG. 15 is a schematic view showing the main software configuration ofthe terminal according to the second embodiment.

FIG. 16 is a schematic view showing the configuration of a GPS deviceaccording to the second embodiment.

FIG. 17 is a schematic view showing the main software configuration ofthe terminal according to the second embodiment.

FIG. 18 is a view illustrative of an estimated frequency calculationprogram according to the second embodiment.

FIG. 19A is a view illustrative of division of a C/A code forillustrating a multiple division search program according to the secondembodiment, and FIG. 19B is a view showing the relationship between aphase and a correlation value for illustrating the multiple divisionsearch program.

FIG. 20 is a view illustrative of a first phase determination programaccording to the second embodiment.

FIG. 21A is a view showing an example of the relationship between aphase and a correlation value for illustrating a first positioning phasecalculation program according to the second embodiment, and FIG. 21B isa view showing another example of the relationship between a phase and acorrelation value.

FIG. 22A is a view illustrative of the process of a first trackingprogram according to the second embodiment, and FIG. 22B is a viewillustrative of the process of the first tracking program.

FIG. 23 is a schematic view showing a positioning method according tothe second embodiment.

FIG. 24A is a view illustrative of the process of a second trackingprogram according to the second embodiment, and FIG. 24B is a viewillustrative of the process of the second tracking program.

FIG. 25 is a schematic flowchart showing an operation example of theterminal according to the second embodiment.

FIG. 26 is a schematic view showing a related art.

DETAILED DESCRIPTION OF THE EMBODIMENT

The invention enables accurate positioning even when the signal strengthof a satellite radio wave is extremely low.

According to one embodiment of the invention, there is provided apositioning device comprising:

a peak frequency determination section which determines a peak frequencywhich is a reception frequency corresponding to a maximum correlationvalue of a specific positioning base code replica and a positioning basecode carried on a radio wave from a specific transmission source;

a reference frequency calculation section which calculates a lowfrequency which is a frequency lower than the peak frequency and a highfrequency which is a frequency higher than the peak frequency;

a reference correlation value calculation section which calculates thecorrelation value corresponding to the low frequency and the correlationvalue corresponding to the high frequency;

a corrected peak frequency calculation section which calculates acorrected peak frequency based on the correlation value corresponding tothe peak frequency, the peak frequency, the correlation valuecorresponding to the low frequency, the low frequency, the correlationvalue corresponding to the high frequency, and the high frequency; and

a radio wave reception section which receives the radio wave using thecorrected peak frequency.

According to this embodiment, since the positioning device includes thepeak frequency determination section, the positioning device candetermine the peak frequency.

Since the positioning device includes the corrected peak frequencycalculation section, the positioning device can calculate the correctedpeak frequency based on a first point specified by the correlation valuecorresponding to the peak frequency and peak frequency, a second pointspecified by the correlation value corresponding to the low frequencyand the low frequency, and a third point specified by the correlationvalue corresponding to the high frequency and the high frequency.

When the phase of the positioning base code replica is fixed, a graphindicating the relationship between the correlation value and thereception frequency (IF carrier frequency) theoretically forms anisosceles triangle having a point corresponding to the maximumcorrelation value as the vertex. The first point is positioned near thevertex of the isosceles triangle, and the second and third points arepositioned on different oblique sides. Since one of the second and thirdpoints is positioned on the same oblique side as the first point, thegradient of the oblique side can be determined. If the gradient of oneoblique side of an isosceles triangle can be determined, the gradient ofthe other oblique side can also be determined. The intersection of thetwo oblique sides is the vertex. The frequency corresponding to thevertex is the corrected peak frequency.

As described above, even if the estimated IF carrier frequency cannot beappropriately determined when the signal strength of the satellite radiowave is extremely low, one peak frequency necessarily exists. If thepeak frequency is determined, the corrected peak frequency can becalculated by the corrected peak frequency calculation section withoutbeing limited to the search width of the frequency search.

Since the positioning device includes the radio wave reception section,the positioning device can receive the radio wave using the correctedpeak frequency. Therefore, the correlation value can be accuratelycalculated, whereby the present position can be accurately calculated.

This makes it possible to accurately locate the position withoutdetermining the IF carrier frequency when the signal strength of thesatellite radio wave is extremely low.

The positioning device according to this embodiment may comprise:

a reception frequency control section which controls the receptionfrequency so that a coherent value of the positioning base code replicaand the positioning base code is maximized.

According to this feature, since the positioning device includes thereception frequency control section, the positioning device can controlthe reception frequency so that the coherent value of the positioningbase code replica and the positioning base code is maximized.

This makes it possible to continuously bring the reception frequencyclose to the IF carrier frequency of the radio wave when the signalstrength of the radio wave is within a specific strength range.

In the positioning device according to this embodiment,

the corrected peak frequency calculation section and the receptionfrequency control section may operate in parallel.

According to this feature, the positioning device can control thereception frequency so that the coherent value of the positioning basecode replica and the positioning base code is maximized when the signalstrength of the radio wave is higher than a specific strength range. Thepositioning device can receive the radio wave using the corrected peakfrequency when the signal strength of the radio wave is lower than aspecific strength.

Therefore, the position can be continuously and accurately located whenthe signal strength has transitioned from a value greater than aspecific strength to a value smaller than the specific strength.

In the positioning device according to this embodiment,

the transmission source may be a positioning satellite.

According to another embodiment of the invention, there is provided apositioning device comprising:

a first correlation value calculation section which performs acorrelation process of a specific positioning base code replica and apositioning base code and calculates a correlation value in units offirst sampling phases which are phases at intervals of a first dividedphase width which is a phase width obtained by equally dividing a phaserange specified by a base unit of a positioning base code formed of aplurality of base units from a transmission source into at least threesections;

a first phase determination section which determines a first phase whichis a sampling phase corresponding to the maximum correlation value;

a first positioning phase calculation section which calculates a firstpositioning phase used for positioning based on three consecutive firstsampling phases including the first phase and the correlation valuescorresponding to the three consecutive first sampling phases includingthe first phase; and

a first located position calculation section which calculates a presentlocated position based on the first positioning phases corresponding tothree or more of the transmission sources.

According to this embodiment, since the positioning device includes thefirst correlation value calculation section, the positioning device cancalculate the correlation values of at least three first sampling phasesin units of base units. Since the positioning device includes the firstphase determination section, the positioning device can determine thefirst phase. Since the positioning device includes the first positioningphase calculation section, the positioning device can determine thefirst positioning phase. Since the positioning device includes thelocated position calculation section, the positioning device cancalculate the located position. As described above, the correlationvalues of the phases EARLY and LATE phases may become equal at aplurality of positions in a weak electric field. On the other hand, onlyone first phase corresponds to the maximum correlation value. Therefore,the true phase exists in the range of the first divided phase width withrespect to the first phase.

Since the graph of the correlation value forms an approximate isoscelestriangle near the first phase even in a weak electric field, the firstpositioning phase, which is the phase corresponding to the vertex of theisosceles triangle, can be calculated from three sampling phasesincluding the first phase and the corresponding correlation values. Thefirst positioning phase is closer to the true phase than the firstphase.

This enables the phase of the received positioning base code to beaccurately estimated even in a weak electric field in which the signalstrength is extremely low.

The positioning device according to this embodiment may comprise:

a reception strength range determination section which determineswhether or not a strength of a radio wave which carries the positioningbase code is within a predetermined reception strength range;

a second correlation value calculation section which performs thecorrelation process of the positioning base code replica and thepositioning base code and calculates the correlation value in units ofsecond sampling phases which are phases at intervals of a second dividedphase width obtained by equally dividing a phase range specified by thebase unit by the second divided phase width which is smaller than thefirst divided phase width based on the determination results of thereception strength range determination section;

a second phase determination section which determines a second phasewhich is a phase of the positioning base code replica corresponding tothe maximum correlation value;

a second positioning phase calculation section which calculates a secondpositioning phase used for positioning based on three consecutivesampling phases including the second phase and the correlation valuescorresponding to the three consecutive sampling phases including thesecond phase; and

a second located position calculation section which calculates thepresent located position based on the second positioning phasescorresponding to three or more of the transmission sources.

According to this feature, since the positioning device includes thesecond correlation value calculation section, the positioning device canperform the correlation process of the positioning base code replica andthe positioning base code in units of second sampling phases andcalculate the correlation values.

Since the positioning device includes the second phase determinationsection, the positioning device can determine the second phase. Sincethe positioning device includes the second positioning phase calculationsection, the positioning device can calculate the second positioningphase. Therefore, the second positioning phase is closer to the truephase than the first positioning phase.

This enables the phase of the received positioning base code to beaccurately estimated even in a weak electric field in which the signalstrength is extremely low to a further extent.

In the positioning device according to this embodiment,

the transmission source may be a positioning satellite;

the positioning base code may be a clear and acquisition or coarse andaccess (C/A) code; and

the base unit may be a chip forming the C/A code.

According to a further embodiment of the invention, there is provided apositioning control method comprising:

a peak frequency determination step of determining a peak frequencywhich is a reception frequency corresponding to a maximum correlationvalue of a specific positioning base code replica and a positioning basecode carried on a radio wave from a specific transmission source;

a reference frequency calculation step of calculating a low frequencywhich is a frequency lower than the peak frequency and a high frequencywhich is a frequency higher than the peak frequency;

a reference correlation value calculation step of calculating thecorrelation value corresponding to the low frequency and the correlationvalue corresponding to the high frequency;

a corrected peak frequency calculation step of calculating a correctedpeak frequency based on the correlation value corresponding to the peakfrequency, the peak frequency, the correlation value corresponding tothe low frequency, the low frequency, the correlation valuecorresponding to the high frequency, and the high frequency; and

a radio wave reception step of receiving the radio wave using thecorrected peak frequency.

This makes it possible to accurately locate the position withoutdetermining the IF carrier frequency when the signal strength of thesatellite radio wave is extremely low.

According to a further embodiment of the invention, there is provided apositioning control program causing a computer to execute:

a peak frequency determination step of determining a peak frequencywhich is a reception frequency corresponding to a maximum correlationvalue of a specific positioning base code replica and a positioning basecode carried on a radio wave from a specific transmission source;

a reference frequency calculation step of calculating a low frequencywhich is a frequency lower than the peak frequency and a high frequencywhich is a frequency higher than the peak frequency;

a reference correlation value calculation step of calculating thecorrelation value corresponding to the low frequency and the correlationvalue corresponding to the high frequency;

a corrected peak frequency calculation step of calculating a correctedpeak frequency based on the correlation value corresponding to the peakfrequency, the peak frequency, the correlation value corresponding tothe low frequency, the low frequency, the correlation valuecorresponding to the high frequency, and the high frequency; and

a radio wave reception step of receiving the radio wave using thecorrected peak frequency.

According to a further embodiment of the invention, there is provided acomputer-readable recording medium having recorded thereon a positioningcontrol program which causes a computer to execute:

a peak frequency determination step of determining a peak frequencywhich is a reception frequency corresponding to a maximum correlationvalue of a specific positioning base code replica and a positioning basecode carried on a radio wave from a specific transmission source;

a reference frequency calculation step of calculating a low frequencywhich is a frequency lower than the peak frequency and a high frequencywhich is a frequency higher than the peak frequency;

a reference correlation value calculation step of calculating thecorrelation value corresponding to the low frequency and the correlationvalue corresponding to the high frequency;

a corrected peak frequency calculation step of calculating a correctedpeak frequency based on the correlation value corresponding to the peakfrequency, the peak frequency, the correlation value corresponding tothe low frequency, the low frequency, the correlation valuecorresponding to the high frequency, and the high frequency; and

a radio wave reception step of receiving the radio wave using thecorrected peak frequency.

According to a further embodiment of the invention, there is provided apositioning control method comprising:

a first correlation value calculation step of performing a correlationprocess of a specific positioning base code replica and a positioningbase code and calculating a correlation value in units of first samplingphases which are phases at intervals of a first divided phase widthwhich is a phase width obtained by equally dividing a phase rangespecified by a base unit of a positioning base code formed of aplurality of base units from a transmission source into at least threesections;

a first phase determination step of determining a first phase which is asampling phase corresponding to the maximum correlation value;

a first positioning phase calculation step of calculating a firstpositioning phase used for positioning based on three consecutive firstsampling phases including the first phase and the correlation valuescorresponding to the three consecutive first sampling phases includingthe first phase; and

a first located position calculation step of calculating a presentlocated position based on the first positioning phases corresponding tothree or more of the transmission sources.

According to a further embodiment of the invention, there is provided apositioning control program causing a computer to execute:

a first correlation value calculation step of performing a correlationprocess of a specific positioning base code replica and a positioningbase code and calculating a correlation value in units of first samplingphases which are phases at intervals of a first divided phase widthwhich is a phase width obtained by equally dividing a phase rangespecified by a base unit of a positioning base code formed of aplurality of base units from a transmission source into at least threesections;

a first phase determination step of determining a first phase which is asampling phase corresponding to the maximum correlation value;

a first positioning phase calculation step of calculating a firstpositioning phase used for positioning based on three consecutive firstsampling phases including the first phase and the correlation valuescorresponding to the three consecutive first sampling phases includingthe first phase; and

a first located position calculation step of calculating a presentlocated position based on the first positioning phases corresponding tothree or more of the transmission sources.

According to a further embodiment of the invention, there is provided acomputer-readable recording medium having recorded thereon a positioningcontrol program which causes a computer to execute:

a first correlation value calculation step of performing a correlationprocess of a specific positioning base code replica and a positioningbase code and calculating a correlation value in units of first samplingphases which are phases at intervals of a first divided phase widthwhich is a phase width obtained by equally dividing a phase rangespecified by a base unit of a positioning base code formed of aplurality of base units from a transmission source into at least threesections;

a first phase determination step of determining a first phase which is asampling phase corresponding to the maximum correlation value;

a first positioning phase calculation step of calculating a firstpositioning phase used for positioning based on three consecutive firstsampling phases including the first phase and the correlation valuescorresponding to the three consecutive first sampling phases includingthe first phase; and

a first located position calculation step of calculating a presentlocated position based on the first positioning phases corresponding tothree or more of the transmission sources.

Preferred embodiments of the invention are described below in detailwith reference to the drawings and the like.

The following embodiments illustrate specific preferred examples of theinvention and are provided with various technologically preferredlimitations. Note that the scope of the invention is not limited tothese embodiments unless there is a description limiting the invention.

Two embodiment are generally described below. Each embodiment includescommon items. Note that the common items are repeatedly described inorder to clarify that the terminal according to each embodiment can beindependently configured.

First Embodiment

FIG. 1 is a schematic view showing a terminal 1020 and the likeaccording to a first embodiment.

As shown in FIG. 1, the terminal 1020 receives radio waves S1, S2, S3,and S4 from GPS satellites (positioning satellites) 12 a, 12 b, 12 c,and 12 d, for example. The radio waves S1 and the like exemplify a radiowave. The GPS satellites 12 a and the like exemplify a transmissionsource.

Various codes are carried on the radio waves S1 and the like. A C/A codeis one of such codes. The C/A code includes 1023 chips. The C/A code isa signal having a bit rate of 1.023 Mbps and a bit length of 1023 bits(=1 msec). The C/A code exemplifies a positioning base code. Theterminal 1020 exemplifies a positioning device which locates the presentposition.

The terminal 1020 is provided in an automobile 1015, and locates thepresent position while being moved along with movement of the automobile1015.

The terminal 1020 receives C/A codes from three or more different GPSsatellites 12 a and the like to locate the present position, forexample.

The terminal 1020 determines the GPS satellite corresponding to thereceived C/A code. The terminal 1020 calculates the phase of thereceived C/A code (hereinafter called “code phase”) by a correlationprocess. The terminal 1020 calculates the distance (hereinafter called“pseudo-range”) between each of the GPS satellites 12 a and the like andthe terminal 1020 using the code phase. The terminal 1020 calculates(locates) the present position based on the position of each of the GPSsatellites 12 a and the like in the satellite orbit at the present timeand the pseudo-range.

Since the C/A code is carried on the radio waves S1 and the like, theaccuracy of the code phase calculated by the correlation processdeteriorates if the reception frequency when the terminal 1020 receivesthe radio waves S1 and the like is inaccurate. Since the GPS satellites12 a and the like move in the satellite orbit, the reception frequencycontinuously changes. When the signal strength of the radio waves S1 andthe like is high, the frequencies can be continuously synchronized by aPLL utilizing the radio waves S1 and the like.

However, the PLL does not effectively function when the signal strengthof the radio waves S1 and the like is extremely low. Moreover, it isdifficult to precisely estimate the IF carrier frequency of the radiowaves S1 and the like when the signal strength of the radio waves S1 andthe like is extremely low.

On the other hand, the terminal 1020 can accurately locate the presentposition as described below without estimating the IF carrier frequencywhen the signal strength of the radio waves S1 and the like is extremelylow.

The terminal 1020 is a portable telephone, a personal handy-phone system(PHS), a personal digital assistance (PDA), or the like. Note that theterminal 1020 is not limited thereto.

The number of GPS satellites 12 a and the like is not limited to four,and may be three or five or more.

(Main Hardware Configuration of Terminal 1020)

FIG. 2 is a schematic view showing the main hardware configuration ofthe terminal 1020.

As shown in FIG. 2, the terminal 1020 includes a computer, and thecomputer includes a bus 1022. A central processing unit (CPU) 1024, astorage device 1026, and the like are connected with the bus 1022. Thestorage device 1026 is a random access memory (RAM), a read-only memory(ROM), or the like.

An external storage device 1028 is also connected with the bus 1022. Theexternal storage device 1028 is a hard disk drive (HDD) or the like.

A power supply device 1030, an input device 1032, a GPS device 1034, adisplay device 1047, and a clock 1048 are also connected with the bus1022.

(Configuration of GPS Device 1034)

FIG. 3 is a schematic view showing the configuration of the GPS device1034.

As shown in FIG. 3, the GPS device 1034 includes an RF section 1035 anda baseband section 1036.

The RF section 1035 receives the radio waves S1 and the like through anantenna 1035 a. An LNA 1035 b (amplifier) amplifies the signal such asthe C/A code carried on the radio wave S1. A mixer 1035 c down-convertsthe frequency of the signal to the IF carrier frequency. A quadrature(IQ) detector 1035 d separates the signal. AD converters 1035 e 1 and1035 e 2 convert the separated signals into digital signals.

The baseband section 1036 receives the digitally-converted IF carrierfrequency signals from the RF section 1035.

A correlation section 1037 of the baseband section 1036 performs acoherent process of synchronously accumulating the input digital signalsover 10 milliseconds (ms) and correlating the accumulation result withthe C/A code replica. The correlation section 1037 includes an NCO 1038,a code generator 1039, and a correlator 1040. The code generator 1039generates the C/A code replica at the timing of a clock signal generatedby the NCO 1038. The correlator 40 correlates the C/A code with the C/Acode replica to determine the phase and calculate the correlation value.The frequency and the phase of the C/A code replica may be set in thecorrelation section 1037.

A signal accumulator 1041 performs an incoherent process of accumulatingthe correlation values output from the correlation section 1037.

A code phase detector 1042 detects the code phase from the value outputfrom the correlation section 1037 and the value output from the signalaccumulator 1041.

As described above, the correlation process is made up of the coherentprocess and the incoherent process.

The coherent process is a process in which the correlation section 1037correlates the received C/A code with the C/A code replica.

For example, when the coherent time is 20 msec, the correlation value ofthe C/A code synchronously accumulated over 20 msec and the C/A codereplica and the like are calculated. The correlated phase and thecorrelation value are output as a result of the coherent process.

The incoherent process is a process in which the incoherent value iscalculated by accumulating the correlation values as the coherentresults.

The phase output by the coherent process and the incoherent value areoutput as a result of the correlation process. The correlation value Pis the incoherent value.

When the signal strength of the radio waves S1 and the like issufficiently high, a phase detector 1043 can acquire phase informationfrom the correlator 1040 and supply the phase information to the NCO1038 to form a PLL. As a result, the C/A code replica can be generatedat a frequency in synchronization with the IF carrier frequency. In moredetail, the reception frequency is controlled so that the correlationvalue P is maximized.

The phase detector 1043, the correlator 1040, and the NCO 1038 exemplifya reception frequency control section.

(Main Software Configuration of Terminal 1020)

FIG. 4 is a schematic view showing the main software configuration ofthe terminal 1020.

As shown in FIG. 4, the terminal 1020 includes a control section 100which controls each section, a GPS section 1102 corresponding to the GPSdevice 1034 shown in FIG. 2, a clock section 1104 corresponding to theclock 1048, a first storage section 1110 which stores various programs,and a second storage section 1150 which stores various types ofinformation.

As shown in FIG. 4, the terminal 1020 stores satellite orbitalinformation 1152 in the second storage section 1150. The satelliteorbital information 1152 includes an almanac 1152 a and an ephemeris1152 b. The almanac 1152 a is information indicating the approximateorbits of all of the GPS satellites 12 a and the like. The ephemeris1152 b is information indicating the precise orbit of each of the GPSsatellites 12 a and the like.

The terminal 1020 uses the almanac 1152 a and the ephemeris 1152 b forpositioning.

As shown in FIG. 4, the terminal 1020 stores initial positioninformation 1154 in the second storage section 1150. The initialposition information 1154 is information indicating the present initialposition P0 of the terminal 1020. The initial position QA0 is thepreceding located position, for example.

As shown in FIG. 4, the terminal 1020 stores an observable satellitecalculation program 1112 in the first storage section 1110. Theobservable satellite calculation program 1112 is a program for causingthe control section 1100 to generate observable satellite information1156 indicating the GPS satellites 12 a and the like which can beobserved from the initial position QA0 at the present time measured bythe clock section 1104 referring to the almanac 1152 a.

The control section 1100 stores the generated observable satelliteinformation 1156 in the second storage section 1150.

As shown in FIG. 4, the terminal 1020 stores a first estimated frequencycalculation program 1114 in the first storage section 1110. The firstestimated frequency calculation program 1114 is a program for causingthe control section 1100 to calculate a first estimated frequency awhich is an estimated value of the IF carrier frequency of each of theradio waves S1 and the like. The first estimated frequency a is anestimated value of the IF carrier frequency of the radio wave S1 whenthe terminal 1020 receives the radio wave S1 from the GPS satellite 12 aat the present time, for example.

FIG. 5 is a view illustrative of the first estimated frequencycalculation program 1114.

As shown in FIG. 5, the first estimated frequency α is a frequencyobtained by adding a Doppler shift H2 to a transmission frequency H1.The transmission frequency H1 is a known value determined by thefrequency (e.g. 1.5 GHz) when the radio waves S1 and the like aretransmitted from the GPS satellites 12 a and the like and thedownconversion rate of the mixer 35 c. The Doppler shift H2 is afrequency shift caused by the relative movement of the GPS satellites 12a and the like and the terminal 1020, and always changes. The Dopplershift H2 may be calculated from the initial position P0 of the terminal1020 and the ephemeris 1152 b.

The control section 1100 stores first estimated frequency information1158 indicating the first estimated frequency a in the second storagesection 1150.

However, since the position of the terminal 1020 is the initial positionQA0 instead of the accurate present position, and the GPS satellites 12a and the like and the terminal 1020 always move relatively, thecalculated Doppler shift H1 may differ from the true Doppler shift.

Therefore, the first estimated frequency α generally differs from thetrue IF carrier frequency.

As shown in FIG. 4, the terminal 1020 stores a first correlation program1116 in the first storage section 1110. The first correlation program1116 is a program for causing the control section 1100 to calculate thecorrelation value of the C/A code received from the GPS satellites 12 aand the like and the C/A code replica, and calculate a first phase CPA1which is the phase (code phase) of the C/A code.

The first phase CPA1 is the phase of the C/A code and is also the phaseof the C/A code replica.

FIG. 6A, FIG. 6B and FIG. 6C are views illustrative of the firstcorrelation program 1116.

As shown in FIG. 6A, the control section 1100 equally divides one chipof the C/A code using the baseband section 1036, and performs thecorrelation process, for example. One chip of the C/A code is equallydivided into 32 sections, for example. Specifically, the control section1100 performs the correlation process at intervals of the phase width of1/32nd of a chip (first phase width W1). The phases at intervals of thefirst phase width W1 when the control section 1100 performs thecorrelation process are called first sampling phases SC1.

The first phase width W1 is specified as a phase width which allowsdetection of the maximum correlation value Pmax when the strength of thesignal input to the antenna 1035 a is −155 dBm or more. A simulationrevealed that the maximum correlation value Pmax can be detected whenthe signal strength is −155 dBm or more by using a phase width of 1/32ndof a chip, even if the electric field is weak.

As shown in FIG. 6B, the control section 1100 performs the correlationprocess while changing the frequency in 100 Hz units within the range ofthe estimated frequency α±100 kHz. The control section 1100 changes thecode phase CP by the first phase width W1 in frequency units, andidentifies the frequency and the code phase which allow calculation ofthe maximum correlation value Pmax.

The control section 1100 changes the C/A code replica from the chip 0 tothe chip 1023 when starting positioning.

When the control section 1100 has determined the code phase and thefrequency corresponding to the maximum correlation value Pmax, thecontrol section 1100 searches for the signals S1 and the like around thecode phase and the frequency corresponding to the maximum correlationvalue Pmax within a range smaller than that when starting positioning.For example, the control section 1100 searches for the signals S1 andthe like within the phase range of ±256 chips around the firstpositioning phase CPA1 which has been calculated. The control section1100 searches for the frequency in 100 Hz units within the range of ±1.0kHz around the frequency corresponding to the maximum correlation valuePmax. This condition is called a first tracking condition.

As shown in FIG. 6C, the correlation values P corresponding to phases C1to C64 of two chips are output from the baseband section 1036. Each ofthe phases C1 to C64 is the first sampling phase SC1.

The ratio of the correlation value Pmax to the correlation value Pnoiseis called a ratio SNR. The correlation value Pnoise is the signal levelof ambient noise. The correlation value Pmax is the signal level fromthe GPS satellites 12 a and the like.

The ratio SNR1 shown in FIG. 6C is relatively small in a state in whichthe strength of the signals S1 and the like is low.

The control section 1100 determines the first phase CPA1 correspondingto the correlation value Pmax.

The control section 1100 stores first phase information 1160 indicatingthe first phase CPA1 in the second storage section 1150.

The smaller the ratio SNR1, the lower the accuracy of the first phaseCPA1.

The operation of the terminal 1020 based on the first correlationprogram 1116 is called a first correlation process.

As shown in FIG. 4, the terminal 1020 stores a first positioning program1118 in the first storage section 1110. The first positioning program1118 is a program for causing the control section 1100 to locate thepresent position based on the first phases CPA1 corresponding to threeor more GPS satellites 12 a and the like and calculate the locatedposition QA1.

FIG. 7 is a schematic view showing a positioning method.

As shown in FIG. 7, it may be considered that a plurality of C/A codescontinuously line up between the GPS satellite 12 a and the terminal1020, for example. Since the distance between the GPS satellite 12 a andthe terminal 1020 is not necessarily a multiple of the length of the C/Acode, a code fraction C/Aa exists. Specifically, a portion of a multipleof the C/A code (portion in which n (n is an integer) C/A codes line up)and a fraction portion (code fraction C/Aa) exist between the GPSsatellite 12 a and the terminal 1020. The total length of the portion ofa multiple of the C/A code and the code fraction C/Aa is thepseudo-range. The terminal 1020 locates the position using thepseudo-range.

The position of the GPS satellite 12 a in the orbit can be calculatedusing the ephemeris 1152 b. The portion of a multiple of the C/A codecan be specified by calculating the distance between the position of theGPS satellite 12 a in the orbit and the initial position QA0.

As shown in FIG. 7, the correlation process is performed while movingthe phase of the C/A code replica in the direction indicated by X1, forexample.

The phase of which the correlation value becomes maximum is the codefraction C/Aa. The code fraction C/Aa is the first phase CPA1.

The control section 1100 calculates the pseudo-range between each of theGPS satellites 12 a and the like and the terminal 1020 based on thefirst phases CPA1 corresponding to three or more GPS satellites 12 a andthe like. The position of each of the GPS satellites 12 a and the likein the orbit is calculated using the ephemeris 1152 b. The controlsection 1100 locates the present position based on the position of eachof three or more GPS satellites 12 a and the like in the orbit and thepseudo-range, and calculates the located position QA1.

The control section 1100 stores first located position information 1162indicating the located position QA1 in the second storage section 1150.

As shown in FIG. 4, the terminal 1020 stores a located position outputprogram 1120 in the first storage section 1110. The located positionoutput program 1120 is a program for causing the control section 1100 todisplay the located position QA1 or a located position QA2 describedlater on the display device 1047.

As shown in FIG. 4, the terminal 1020 stores a second correlationprogram 1122 in the first storage section 1110. The second correlationprogram 1122 is a program for causing the control section 1100 toperform the correlation process to calculate the correlation value P andthe code phase CP.

The control section 1100 stores second correlation information 1164indicating the correlation value P and the code phase CP in the secondstorage section 1150.

As shown in FIG. 4, the terminal 1020 stores a peak frequencydetermination program 1124 in the first storage section 1110. The peakfrequency determination program 1124 and the control section 1100exemplify a peak frequency determination section.

FIG. 8 is a view illustrative of the peak frequency determinationprogram 1124.

As shown in FIG. 8, the control section 1100 determines the frequencycorresponding to the maximum correlation value Pmax as a peak frequencyFA0. The peak frequency FA0 exemplifies a peak frequency.

Since the peak frequency FA0 is obtained by causing the terminal 1020 tosearch for the frequency at intervals of 100 Hz, the peak frequency FA0differs from the true IF carrier frequency of the received radio wavesS1 and the like by about 50 Hz or less.

The control section 1100 stores peak frequency information 1166indicating the peak frequency FA0 in the second storage section 1150.

As shown in FIG. 4, the terminal 1020 stores a reference frequencycalculation program 1126 in the first storage section 1110. Thereference frequency calculation program 1126 and the control section1100 exemplify a reference frequency calculation section.

The control section 1100 calculates a frequency FA1 lower than the peakfrequency FA0 by 100 Hz and a frequency FA2 higher than the peakfrequency FA0 by 100 Hz based on the reference frequency calculationprogram 1126. The control section 1100 stores reference frequencyinformation 1168 indicating the frequency FA1 and the frequency FA2 inthe second storage section 1150. The frequency FA1 exemplifies a lowfrequency. The frequency FA2 exemplifies a high frequency.

The frequency FA1 and the frequency FA1 are determined so that thedifference between the peak frequency FA0 and the frequency FA1 becomesequal to the difference between the peak frequency FA0 and the frequencyFA2. In the first embodiment, the difference in frequency is set at 100Hz.

Note that the difference in frequency is not limited to 100 Hz.

As shown in FIG. 4, the terminal 1020 stores a reference correlationvalue calculation program 1128 in the first storage section 1110. Thereference correlation value calculation program 1128 and the controlsection 1100 exemplify a reference correlation value calculationsection.

The control section 1100 calculates a correlation value PA1corresponding to the frequency FA1 and a correlation value PA2corresponding to the frequency FA2 based on the reference correlationvalue calculation program 1128. In more detail, the control section 1100calculates the correlation value PA1 and the correlation value PA2referring to the second correlation information 1164.

The control section 1100 stores reference correlation value information1170 indicating the correlation value PA1 and the correlation value PA2in the second storage section 1150.

As shown in FIG. 4, the terminal 1020 stores a second estimatedfrequency calculation program 1130 in the first storage section 110. Thesecond estimated frequency calculation program 1130 is a program forcausing the control section 1100 to calculate a second estimatedfrequency Fr based on the peak frequency FA0, the correlation peak valuePmax (PA0), the frequency FA1, the correlation value PA1, the frequencyFA2, and the correlation value PA2. The second estimated frequency Frexemplifies a corrected peak frequency. The second estimated frequencycalculation program 1130 and the control section 1100 exemplify acorrected peak frequency calculation section.

FIG. 9A, FIG. 9B, FIG. 10A and FIG. 10B are views illustrative of thesecond estimated frequency calculation program 1130.

As shown in FIG. 9A, FIG. 9B, FIG. 10A and FIG. 10B, a graph indicatingthe correlation value P and the frequency F forms an isosceles triangle.

As shown in FIGS. 9A and 10A, a point GA0 is specified by the peakfrequency FA0 and the correlation peak value PA0. A point GA1 isspecified by the frequency FA1 and the correlation value PA1. A pointGA2 is specified by the frequency FA2 and the correlation value PA2.

As shown in FIGS. 9A and 9B, when the correlation value PA1 is smallerthan the correlation value PA2, the point GA0 and the point GA1 exist ona single straight line with a gradient of a (a is a positive number).The straight line connecting the point GA0 and the point GA1 is astraight line LA1.

The point GA2 exists on a straight line with a gradient of −a. Thestraight line having a gradient of −a and passing through the point GA2is a straight line LA2.

The intersection of the straight line LA1 and the straight line LA2 isthe vertex H of the isosceles triangle. The frequency corresponding tothe vertex H is the second estimated frequency Fr. The unknown numbersFr and Pr and the gradient a can be calculated by solving thesimultaneous equations 1 shown in FIG. 9B.

As shown in FIGS. 10A and 10B, when the correlation value PA1 is greaterthan the correlation value PA2, the point GA0 and the point GA2 exist ona single straight line with a gradient of −a (a is a positive number).The straight line connecting the point GA0 and the point GA2 is astraight line LA2.

The point GA1 exists on a straight line with a gradient a. The straightline having a gradient of a and passing through the point GA 1 is astraight line LA1.

The intersection of the straight line LA1 and the straight line LA2 isthe vertex H of the isosceles triangle. The frequency corresponding tothe vertex H is the second estimated frequency Fr. The unknown numbersFr and Pr and the gradient a can be calculated by solving thesimultaneous equations 2 shown in FIG. 10B.

When the correlation value PA1 is equal to the correlation value PA2,the peak frequency FA0 is the second estimated frequency Fr.

The control section 1100 stores second estimated frequency information1172 indicating the second estimated frequency Fr in the second storagesection 1150.

Since the second estimated frequency Fr is not affected by a limitationof 100 Hz which is the search step for the frequency F, the secondestimated frequency Fr is information with high accuracy. Specifically,the second estimated frequency Fr is closer to the true IF carrierfrequency than the peak frequency FA0.

As shown in FIG. 4, the terminal 1020 stores a second phasedetermination program 1132 in the first storage section 1110. The secondphase determination program 1132 is a program for causing the controlsection 1100 to receive the radio waves S1 and the like using the secondestimated frequency Fr, perform the correlation process, and calculate asecond phase CPA2 for positioning. The second phase determinationprogram 1132 and the control section 1100 exemplify a radio wavereception section.

FIG. 11 is a view illustrative of the second phase determination program1132.

The ratio SNR2 in the correlation graph shown in FIG. 11 is greater thanthe ratio SNR1 in the graph shown in FIG. 6C. This is because the secondestimated frequency Fr is very close to the true IF carrier frequency.

Therefore, the second phase CPA2 which is the phase corresponding to themaximum correlation value Pmax is phase information with high accuracy.

The control section 1100 stores second phase information 1174 indicatingthe second phase CPA2 in the second storage section 1150.

The operation of the terminal 1020 based on the second correlationprogram 1122, the peak frequency determination program 1124, thereference frequency calculation program 1126, the reference correlationvalue calculation program 1128, the second estimated frequencycalculation program 1130, and the second phase determination program1132 is called a second correlation process.

As shown in FIG. 4, the terminal 1020 stores a second positioningprogram 1134 in the first storage section 110. The second positioningprogram 1134 is a program for causing the control section 1100 to locatethe position based on the second phases CPA2 for three or more GPSsatellites 12 a and the like and calculate the located position QA2.

The control section 1100 stores second located position information 1176indicating the located position QA2 in the second storage section 1150.

As shown in FIG. 4, the terminal 1020 stores a signal strengthevaluation program 1136 in the first storage section 1110.

The signal strength evaluation program 1136 is a program for evaluatingthe strength SP of the signal input to the antenna 1035 a. The strengthSP of the signal input to the antenna 1035 a may be estimated from thecorrelation value.

The control section 1100 performs the first correlation process when thesignal strength SP is −138 dBm or more, and calculates the locatedposition QA 1.

The control section 1100 performs the second correlation process whenthe signal strength SP is −142 dBm or less, and calculates the locatedposition QA2.

The control section 1100 performs the first correlation process and thesecond correlation process in parallel when the signal strength SP is−142 dBm or more and less than −138 dBm. The control section 1100calculates the located position QA1 using the first phase CPA 1.

The terminal 1020 is configured as described above.

The terminal 1020 can determine the peak frequency FA0 (see FIG. 4), asdescribed above.

The terminal 1020 can calculate the second estimated frequency Fr (seeFIG. 4).

When the phase of the C/A code replica is fixed, the graph indicatingthe relationship between the correlation value and the receptionfrequency (IF carrier frequency) forms an isosceles triangle having apoint corresponding to the maximum correlation value as the vertex, asshown in FIG. 9A and FIG. 9B. The point GA0 corresponding to the peakfrequency FA0 is positioned near the vertex H, and the points GA1 andGA2 respectively corresponding to the frequencies FA1 and FA2 lower orhigher than the peak frequency FA0 are positioned on different obliquesides. Since one of the points GA1 and GA2 is positioned on the sameoblique side as the point GA1, the gradient a of the oblique side can bedetermined. If the gradient of one oblique side of the isoscelestriangle can be determined, the gradient of the other oblique side canalso be determined. The intersection of the two oblique sides is thevertex H. The frequency corresponding to the vertex H is the secondestimated frequency Fr.

When the signal strength of the radio waves S1 and the like is extremelylow, one peak frequency FA0 necessarily exists even if the estimated IFcarrier frequency cannot be determined, as described above. The secondestimated frequency Fr can be calculated by determining the peakfrequency FA0.

The terminal 1020 can receive the radio waves S1 and the like using thesecond estimated frequency Fr. Therefore, the correlation value P can beaccurately calculated, whereby the present position can be accuratelycalculated.

This makes it possible to accurately locate the position withoutdetermining the IF carrier frequency when the signal strength of thesatellite radio wave is extremely low.

The terminal 1020 can control the reception frequency using the PLL sothat the coherent value of the C/A code replica and the received C/Acode is maximized.

This makes it possible to cause the PLL to effectively function when thesignal strength of the radio waves S1 and the like is within a specificrange, whereby the reception frequency can be continuously brought closeto the IF carrier frequency of the radio waves S1 and the like.

The terminal 1020 can perform the first correlation process and thesecond correlation process in parallel when the signal strength of theradio waves S1 and the like is within a specific range. Therefore, theposition can be continuously and accurately located when the signalstrength SP has transitioned from a value greater than a specificstrength to a value smaller than the specific strength.

The configuration of the terminal 1020 according to the first embodimenthas been described above. An operation example of the terminal 1020 isdescribed below mainly using FIGS. 12 and 13.

FIGS. 12 and 13 are schematic flowcharts showing an operation example ofthe terminal 1020.

The terminal 1020 calculates the estimated frequency a of each of theGPS satellites 12 a and the like from the ephemeris 1152 b and theinitial position QA0 (step ST1 in FIG. 12).

The terminal 1020 performs the first correlation process (step ST2).

The terminal 20 determines the signal strength SP (step ST3).

When the terminal 1020 has determined that the signal strength SP is−138 dBm or more in the step ST3, the terminal 1020 continuouslyperforms the first correlation process (step ST4A). The terminal 1020locates the present position using the first phase CPA 1, and calculatesthe located position QA1 (step ST5A).

The terminal 1020 outputs the located position QA1 (step ST6A).

The terminal 1020 determines whether or not positioning has beenperformed a specific number of times (e.g. 10 times) (step ST7).

When the terminal 1020 has determined that positioning has beenperformed a specific number of times, the terminal 1020 finishes thepositioning operation.

When the terminal 1020 has determined that positioning has not beenperformed a specific number of times, the terminal 1020 executes thestep ST3 and the subsequent steps.

When the terminal 1020 has determined that the signal strength SP is−142 dBm or less in the step ST3, the terminal 1020 stops the firstcorrelation process, and performs the second correlation process (stepST4B).

In the second correlation process, the terminal 1020 determines the peakfrequency FA0 (see FIG. 4) (step ST101 in FIG. 13). The step ST101exemplifies a peak frequency determination step.

The terminal 1020 calculates the frequencies FA1 and FA2 (see FIG. 4)(step ST102). The step ST102 exemplifies a reference frequencycalculation step.

The terminal 1020 calculates the correlation values PA1 and PA2 (seeFIG. 4) (step ST103). The step ST103 exemplifies a reference correlationvalue calculation step.

The terminal 1020 calculates the second estimated frequency Fr (see FIG.4) (step ST104). The step ST104 exemplifies a corrected peak frequencycalculation step.

The terminal 1020 calculates the second phase CPA2, locates the presentposition using the second phase CPA2, and calculates the locatedposition QA2 (step ST5B in FIG. 12).

The terminal 1020 outputs the located position QA2 (step ST6B), andexecutes the step ST7.

When the terminal 1020 has determined that the signal strength SP ismore than −142 dBm and less than −138 dBm in the step ST3, the terminal1020 performs the first correlation process and the second correlationprocess in parallel (step ST4C in FIG. 12).

The terminal 1020 locates the present position using the first phaseCPA1, and calculates the located position QA1 (step ST5C).

The terminal 1020 outputs the located position QA1 (step ST6C), andexecutes the step ST7.

When the terminal 1020 has determined that positioning has not beenperformed a specific number of times, the terminal 1020 executes thestep ST3 and the subsequent steps. When the terminal 1020 has determinedthat the signal strength SP is −138 dBm or less, in the step ST3, theterminal 1020 executes the step ST4B. Since the terminal 1020continuously performs the first correlation process and the secondcorrelation process in parallel, the terminal 1020 can stop the firstcorrelation process and immediately perform the second correlationprocess. This means that the second correlation process is continuouslyperformed in an intermediate state in which the signal strength SP maydecrease to −142 dBm or less (state in which the signal strength SP ismore than −142 dBm and less than −138 dBm) instead of starting thesecond correlation process after the PLL has not functioned in the firstcorrelation process. Therefore, since it is unnecessary to again searchfor the frequency and the phase over a wide range in the secondcorrelation process, the step ST5B and the subsequent steps can bepromptly executed.

The intermediate state (state in which the signal strength SP is morethan −142 dBm and less than −138 dBm) is also a state in which thesignal strength SP may increase to −138 dBm or more. It is possible toimmediately transition to a state in which only the first correlationprocess is executed when the signal strength has increased to −138 dBmor more by continuously performing the first correlation process inadvance.

Second Embodiment

FIG. 14 is a schematic view showing a terminal 2020 and the likeaccording to a second embodiment.

As shown in FIG. 14, the terminal 2020 receives radio waves S1, S2, S3,and S4 from GPS satellites positioning satellites) 12 a, 12 b, 12 c, and12 d, for example. The GPS satellites 12 a and the like exemplify atransmission source.

Various codes are carried on the radio waves S1 and the like. A C/A codeSca is one of such codes. The C/A code Sca is a signal having a bit rateof 1.023 Mbps and a bit length of 1023 bits (=1 msec). The C/A code Scaincludes 1023 chips. The terminal 2020 exemplifies a positioning devicewhich locates the present position, and locates the present positionusing the C/A code. The C/A code Sca exemplifies a positioning basecode. The chip exemplifies a base unit.

As information carried on the radio waves S1 and the like, an almanacSal and an ephemeris Seh can be given. The almanac Sal is informationindicating the approximate satellite orbits of all of the GPS satellites12 a and the like, and the ephemeris Seh is information indicating theprecise satellite orbit of each of the GPS satellites 12 a and the like.The almanac Sal and the ephemeris Seh are generically called anavigation message.

The terminal 2020 receives C/A codes from three or more different GPSsatellites 12 a and the like to locate the present position, forexample.

The terminal 2020 determines the GPS satellite corresponding to thereceived C/A code. The terminal 2020 calculates the distance(hereinafter called “pseudo-range”) between each of the GPS satellites12 a and the like and the terminal 2020 by determining the phase of theC/A code. The terminal 2020 calculates (locates) the present positionbased on the position of each of the GPS satellites 12 a and the like inthe satellite orbit at the present time and the pseudo-range.

The terminal 2020 performs a coherent process and an incoherent processdescribed later in order to determine the phase of the C/A code.

Note that the terminal 2020 may locate the position using a radio wavefrom a portable telephone base station or the like, differing from thisembodiment. The terminal 2020 may locate the position by receiving aradio wave through a local area network (LAN), differing from thisembodiment.

(Main Hardware Configuration of Terminal 2020)

FIG. 15 is a schematic view showing the main hardware configuration ofthe terminal 2020.

As shown in FIG. 15, the terminal 2020 includes a computer, and thecomputer includes a bus 2022. A central processing unit (CPU) 2024, astorage device 2026, and the like are connected with the bus 2022. Thestorage device 2026 is a random access memory (RAM), a read-only memory(ROM), or the like.

An input device 2028, a power supply device 2030, a GPS device 2032, adisplay device 2034, a communication device 2036, and a clock 2038 arealso connected with the bus 2022.

(Configuration of GPS Device 2032)

FIG. 16 is a schematic view showing the configuration of the GPS device2032.

As shown in FIG. 16, the GPS device 2032 includes an RF section 2032 aand a baseband section 2032 b.

The RF section 2032 a receives the radio waves S1 and the like throughan antenna 2033 a. An LNA 2033 b (amplifier) amplifies the signal suchas the C/A code carried on the radio wave S1. A mixer 2033 cdown-converts the frequency of the signal. A quadrature (IQ) detector2033 d separates the signal. A/D converters 2033 e 1 and 2033 e 2convert the separated signals into digital signals.

The baseband section 2032 b receives the digitally-converted signalsfrom the RF section 2032 a, samples and accumulates each chip (notshown) of the signals, and correlates the signals with the C/A code heldby the baseband section 2032 b. The baseband section 2032 b includes 128correlators (not shown) and accumulators (not shown), and can perform acorrelation process at the same time for 128 phases. The correlator is aconfiguration for performing the coherent process described later. Theaccumulator is a configuration for performing the incoherent processdescribed later.

(Main Software Configuration of Terminal 2020)

FIG. 17 is a schematic view showing the main software configuration ofthe terminal 2020.

As shown in FIG. 17, the terminal 2020 includes a control section 2100which controls each section, a GPS section 2102 corresponding to the GPSdevice 2032 shown in FIG. 15, a clock section 2104 corresponding to theclock 2038, and the like.

The terminal 2020 also includes a first storage section 2110 whichstores various programs, and a second storage section 2150 which storesvarious types of information.

As shown in FIG. 17, the terminal 2020 stores a navigation message 2152in the second storage section 2150. The navigation message 2152 includesan almanac 2152 a and an ephemeris 2152 b.

The terminal 2020 uses the almanac 2152 a and the ephemeris 2152 b forpositioning.

As shown in FIG. 17, the terminal 2020 stores an observable satellitecalculation program 2112 in the first storage section 2110. Theobservable satellite calculation program 2112 is a program for causingthe control section 2100 to calculate the observable GPS satellites 12 aand the like based on an initial position QB0 indicated by initialposition information 2156.

In more: detail, the control section 2100 determines the GPS satellites12 a and the like which can be observed at the present time measured bythe clock section 2104 referring to the almanac 2152 a. The initialposition QB0 is the preceding located position, for example.

The control section 2100 stores observable satellite information 2154indicating the observable GPS satellites 12 a and the like in the secondstorage section 2150.

As shown in FIG. 17, the terminal 2020 stores an estimated frequencycalculation program 2114 in the first storage section 2110. Theestimated frequency calculation program 2114 is a program for causingthe control section 2100 to estimate the reception frequency of theradio waves S1 and the like from the GPS satellites 12 a and the like.

FIG. 18 is a view illustrative of the estimated frequency calculationprogram 2114.

As shown in FIG. 18, the control section 2100 adds a Doppler shift H2 toa transmission frequency H1 from the GPS satellites 12 a and the like tocalculate an estimated frequency β. The transmission frequency from theGPS satellites 12 a and the like is known (e.g. 1575.42 MHz).

The Doppler shift H2 occurs due to the relative movement of each of theGPS satellites 12 a and the like and the terminal 2020. The controlsection 2100 calculates the line-of-sight velocity (velocity in thedirection of the terminal 2020) of each of the GPS satellites 12 a andthe like at the present time using the ephemeris 2152 b. The controlsection 2100 calculates the Doppler shift H2 based on the line-of-sightvelocity.

The control section 2100 calculates the estimated frequency β for eachof the GPS satellites 12 a and the like.

The estimated frequency β includes an error of drift of a clock(reference oscillator: not shown) of the terminal 2020. The term “drift”refers to a change in oscillation frequency due to a change intemperature.

Therefore, the control section 2100 searches for the radio waves S1 andthe like around the estimated frequency β over a frequency range with aspecific width. For example, the control section 2100 searches for theradio waves S1 and the like at intervals of 100 Hz within the range from(A−100) kHz to (A+100) kHz.

As shown in FIG. 17, the terminal 2020 stores a multiple division searchprogram 2116 in the first storage section 2110. The multiple divisionsearch program 2116 is a program for causing the control section 2100 tocorrelate the C/A code received from the GPS satellites 12 a and thelike with the C/A code replica generated by the terminal 2020 atintervals of a phase width obtained by equally dividing the phase rangespecified by the chip into at least three sections to calculate thecorrelation value. The multiple division search program 2116 and thecontrol section 2100 exemplify a first correlation value calculationsection. The C/A code replica exemplifies a positioning base codereplica.

FIG. 19A and FIG. 19B are views illustrative of the multiple divisionsearch program 2116.

As shown in FIG. 19A, the control section 2100 equally divides one chipof the C/A code using the baseband section 2032 b, and performs thecorrelation process, for example. One chip of the C/A code is equallydivided into 32 sections, for example. Specifically, the control section1100 performs the correlation process at intervals of a phase width of1/32nd of a chip (first phase width W1). The first phase width W1exemplifies a first divided phase width. The phases at intervals of thefirst phase width W1 when the control section 2100 performs thecorrelation process are called first sampling phases SC1. The firstsampling phase SC1 exemplifies a first sampling phase.

The first phase width W1 is specified as a phase width which allowsdetection of the maximum correlation value Pmax when the signal strengthis −155 dBm or more. A simulation revealed that the maximum correlationvalue Pmax can be detected when the signal strength is −155 dBm or moreby using a phase width of 1/32nd of a chip, even if the electric fieldis weak.

As shown in FIG. 19B, the correlation values P corresponding to phasesC1 to C64 of two chips are output from the baseband section 2032 b. Eachof the phases C1 to C64 is the first sampling phase SC1.

The control section 2100 searches for the first chip to the 1023rd chipof the C/A code based on the multiple division search program 2116, forexample.

The search based on the multiple division search program 2116 is calleda multiple division search.

The correlation process is made up of the coherent process and theincoherent process.

The coherent process is a process in which the baseband section 2032 bcorrelates the received C/A code with the C/A code replica.

For example, when the coherent time is 20 msec, the correlation value ofthe C/A code synchronously accumulated over 20 msec and the C/A codereplica and the like are calculated. The correlated phase and thecorrelation value are output as a result of the coherent process.

The incoherent process is a process in which the incoherent value iscalculated by accumulating the correlation values as the coherentresults.

The phase output by the coherent process and the incoherent value areoutput as a result of the incoherent process. The correlation value P isthe incoherent value.

The control section 2100 stores correlation information 2160 indicatingthe phases C1 to C64 subjected to the correlation process and thecorrelation value P in the second storage section 2150.

The terminal 2020 stores a first phase determination program 2118 in thefirst storage section 2110. The first phase determination program 2118is a program for causing the control section 2100 to determine a firstphase CPB0 which is a phase corresponding to the maximum correlationvalue Pmax. The first phase CPB0 exemplifies a first phase. The firstphase determination program 2118 and the control section 2100 exemplifya first phase determination section.

FIG. 20 shows an example of the first phase determination program 2118.

The correlation information 2160 may be expressed by the graph shown inFIG. 20 (hereinafter called “correlation value graph”).

As shown in FIG. 20, the control section 2100 determines the first phaseCPB0 corresponding to the correlation value Pmax referring to thecorrelation information 2160.

The control section 2100 stores first phase information 2162 indicatingthe first phase CPB0 in the second storage section 2150.

The terminal 2020 stores a first positioning phase calculation program2120 in the first storage section 2110. The first positioning phasecalculation program 2120 is a program for causing the control section2100 to calculate a first positioning phase CPB3 used for positioningbased on three consecutive first sampling phases SC1 including the firstphase CPB0 and the correlation values P corresponding to the three firstsampling phases SC1. The first positioning phase calculation program2120 and the control section 2100 exemplify a first positioning phasecalculation section.

FIG. 21A and FIG. 21B are views illustrative of the first positioningphase calculation program 2120.

FIG. 21A and FIG. 21B are enlarged views showing a portion near thefirst phase CPB0 shown in FIG. 22B.

Even if the signal strength is extremely low, the correlation values Pform an approximate isosceles triangle (shape near the vertex ofapproximate isosceles triangle) within a narrow phase range incoordinates of which the vertical axis indicates the correlation value Pand the horizontal axis indicates the code phase CP.

The two oblique sides of the isosceles triangle can be determined bydetermining three points in the correlation value graph. The phasecorresponding to the vertex is the first positioning phase CPB3.

As shown in FIG. 21A, phases CPB1 and CPB2 continuous with the firstphase CPB0 are used, for example. The phase CPB1 is a phase whichadvances from the first phase CPB0 by 1/32nd of a chip. The phase CPB2is a phase which is delayed from the first phase CPB0 by 1/32nd of achip.

In the correlation value graph, a point GB1 is specified by the firstphase CPB0 and the correlation value PB1. Likewise, a point GB2 isspecified by the first phase CPB1 and the correlation value PB3. A pointGB3 is specified by the first phase CPB2 and the correlation value PB2.

Since the first phase CPB0 is a phase corresponding to the maximumcorrelation value Pmax, the correlation value PB1 (Pmax) correspondingto the first phase CPB0 is greater than the correlation value PB3 of thephase CPB1 and the correlation value PB2 of the phase CPB2.

As shown in FIG. 21A, when the correlation value PB3 of the phase CPB1is smaller than the correlation value PB2 of the phase CPB2, the pointsGB2 and GB1 exist on a single straight line. A straight line LB1 isformed by connecting the points GB2 and GB1. The gradient of thestraight line LB1 is referred to as a (a is a positive number).

The gradient of the other oblique side of the isosceles triangle shownin the correlation value graph is −a. The point GB3 exists on theoblique side with a gradient of −a. A straight line LB2 is specified bythe gradient −a and the point GB3.

A portion near the vertex of the isosceles triangle in the correlationvalue graph is formed by connecting the straight line LB1 and thestraight line LB2. The vertex H can be determined by forming the portionnear the vertex. The phase CPB3 corresponding to the vertex H is thefirst positioning phase CPB3.

As shown in FIG. 21B, when the correlation value PB3 of the phase CPB1is greater than the correlation value PB2 of the phase CPB2, the pointsGB1 and GB3 exist on a single straight line. A straight line LB2 isformed by connecting the points GB1 and GB3. The gradient of thestraight line LB2 is referred to as −a (a is a positive number).

The gradient of the other oblique side of the isosceles triangle shownin the correlation value graph is a. The point GB2 exists on the obliqueside with the gradient a. A straight line LB1 is specified by thegradient a and the point GB2.

A portion near the vertex of the isosceles triangle in the correlationvalue graph is formed by connecting the straight line LB1 and thestraight line LB2. The vertex H can be determined by forming the portionnear the vertex. The phase CPB3 corresponding to the vertex H is thefirst positioning phase CPB3.

The control section 2100 stores first positioning phase information 2166indicating the first positioning phase CPB3 in the second storagesection 2150.

As shown in FIG. 17, the terminal 2020 stores a signal strengthevaluation program 2122 in the first storage section 2110. The signalstrength evaluation program 2122 is a program for causing the controlsection 2100 to determine whether or not the signal strength (radio wavestrength) of the radio waves S1 and the like which carry the C/A code is−155 dBm or more. A range of −155 dBm or more exemplifies apredetermined reception strength range. The signal strength evaluationprogram 2122 and the control section 2100 exemplify a reception strengthrange determination section.

In more detail, the control section 2100 calculates the strength of thesignal input to the antenna 2033 a (see FIG. 16) from the maximumcorrelation value Pmax. Since the relationship between the maximumcorrelation value Pmax and the signal strength is known, the controlsection 2100 can calculate the signal strength input to the antenna 2033a from the maximum correlation value Pmax.

As shown in FIG. 17, the terminal 2020 stores a first tracking program2124 in the first storage section 2110. The first tracking program 2124is a program for causing the control section 2100 to continuouslycalculate the first positioning phase CPB3 when the control section 2100has determined that the radio wave strength is −155 dBm or more based onthe signal strength evaluation program 2122.

FIG. 22A and FIG. 22B are views illustrative of the first trackingprogram 2124.

As shown in FIG. 22A, the control section 2100 performs control similarto the control based on the multiple division search program 2116 basedon the first tracking program 2124 excluding the search initiationphase. Since the first positioning phase CPB3 has been calculated whenperforming control based on the first tracking program 2124, the controlsection 2100 searches for the phase around the first positioning phaseCPB3.

As shown in FIG. 22B, the control section 2100 determines the firstphase CPB0 based on the first tracking program 2124 in the same manneras in control based on the first phase determination program 2118.

The control section 2100 searches for the phase in the range of ±256chips around the first phase CPB3 which has been calculated.

The control section 2100 searches for the frequency in 100 Hz unitswithin the range of ±1.0 kHz around the estimated frequency P.

The control section 2100 calculates the first positioning phase CPB3based on the first phase CPB0 and the phases CPB1 and CPB2 in the samemanner as in control based on the first positioning phase calculationprogram 2120.

The tracking condition of the first tracking program 2124 is called afirst tracking condition.

As shown in FIG. 17, the terminal 2020 stores a first positioningprogram 2126 in the first storage section 2110. The first positioningprogram 2126 is a program for causing the control section 2100 to locatethe present position based on the first positioning phases CPB3corresponding to three or more GPS satellites 12 a and the like andcalculate the located position QB1. The first positioning program 2126and the control section 2100 exemplify a first located positioncalculation section.

FIG. 23 is a schematic view showing a positioning method.

As shown in FIG. 23, it may be considered that n C/A codes continuouslyline up between the GPS satellite 12 a and the terminal 2020, forexample. Since the distance between the GPS satellite 12 a and theterminal 2020 is not necessarily a multiple of the length of the C/Acode, a code fraction C/Aa exists. Specifically, a portion of a multipleof the C/A code and a fraction portion exist between the GPS satellite12 a and the terminal 2020. The total length of the portion of amultiple of the C/A code and the fraction portion is the pseudo-range.The terminal 2020 locates the position using the pseudo-range.

The position of the GPS satellite 12 a in the orbit can be calculatedusing the ephemeris 2152 b. The portion of a multiple of the C/A codecan be specified by calculating the distance between the position of theGPS satellite 12 a in the orbit and the initial position QB0.

As shown in FIG. 23, the correlation process is performed while movingthe phase of the C/A code replica in the direction indicated by X1, forexample.

The phase of which the correlation value becomes maximum is the codefraction C/Aa. The code fraction C/Aa is the first positioning phaseCPB3.

The control section 2100 calculates the pseudo-range between each of theGPS satellites 12 a and the like and the terminal 2020 based on thefirst positioning phases CPB3 corresponding to three or more GPSsatellites 12 a and the like. The position of each of the GPS satellites12 a and the like in the orbit is calculated using the ephemeris 2152 b.The control section 2100 locates the present position based on theposition of each of three or more GPS satellites 12 a and the like inthe orbit and the pseudo-range, and calculates the located position QB1.

The control section 2100 stores first located position information 2166indicating the located position QB1 in the second storage section 2150.

As shown in FIG. 17, the terminal 2020 stores a located position outputprogram 2128 in the first storage section 2110. The located positionoutput program 2128 is a program for causing the control section 2100 todisplay the located position QB1 or a located position QB2 describedlater on the display device 2034.

As shown in FIG. 17, the terminal 2020 stores a second tracking program2130 in the first storage section 2110. The second tracking program 2130is a program for causing the control section 2100 to continuouslycalculate a second positioning phase CPB4 when the control section 2100has determined that the radio wave strength is not −155 dBm or morebased on the signal strength evaluation program 2122.

The operation of the terminal 2020 based on the second tracking program2130 is the same as the operation of the terminal 2020 based on thefirst tracking program 2124 excluding the search phase width.

FIG. 24A and FIG. 24B are views illustrative of the second trackingprogram 2130.

As shown in FIG. 24A, the baseband section 2032 b (see FIG. 16) performsthe correlation process in units of phases (second sampling phase SC2)at intervals of a phase width (second phase width W2) obtained byequally dividing the phase range of two chips into 128 sections. Thismeans that one chip is equally divided into 64 sections. The secondphase width W2 is smaller than the first phase width W1. The secondphase width W2 exemplifies a second divided phase width. The secondsampling phase SC2 exemplifies a second sampling phase.

The second phase width W2 is specified as a phase width which allowsdetection of the maximum correlation value Pmax even when the signalstrength is less than −155 dBm. A simulation revealed that the maximumcorrelation value Pmax can be detected even when the signal strength isless than −155 dBm by using a phase width of 1/64th of a chip.

The control section 2100 searches for the phase in the range of ±128chips around the first positioning phase CPB3 which has been calculated.This code phase search width is smaller than that of the first trackingcondition. This enables the second phase CPB02 and the secondpositioning phase CPB4 described later to be more accurately calculated.

The control section 2100 searches for the frequency in 50 Hz unitswithin the range of ±0.5 kHz around the preceding reception frequency.This frequency search width is smaller than that of the first trackingcondition. This also enables the second phase CPB02 and the secondpositioning phase CPB4 described later to be more accurately calculated.

The tracking condition of the second tracking program 2130 is called asecond tracking condition.

As shown in FIG. 24B, the control section 2100 determines a phase CPB0 scorresponding to the maximum correlation value Pmax, and determines aphase CPB1 s which advances from the phase CPB0 s by 1/64th of a chipand a phase CPB2 s which is delayed from the phase CPB0 s by 1/64th of achip. The control section 2100 calculates the second positioning phaseCPB4 by a process similar to control based on the first tracking program2124.

The control section 2100 stores second positioning phase information2168 indicating the second positioning phase CPB4 in the second storagesection 2150.

As shown in FIG. 17, the terminal 2020 stores a second positioningprogram 2132 in the first storage section 2110. The second positioningprogram 2132 is a program for causing the control section 2100 to locatethe present position based on the second positioning phases CPB4corresponding to three or more GPS satellites 12 a and the like andcalculate the located position QB2. The second positioning program 2132and the control section 2100 exemplify a second located positioncalculation section.

The control section 2100 calculates the pseudo-range between each of theGPS satellites 12 a and the like and the terminal 2020 based on thesecond positioning phase CPB4. The position of each of the GPSsatellites 12 a and the like in the orbit is calculated using theephemeris 2152 b. The control section 2100 locates the present positionbased on the position of each of three or more GPS satellites 12 a andthe like in the orbit and the pseudo-range, and calculates the locatedposition QB2.

The control section 2100 stores second located position information 2170indicating the located position QB2 in the second storage section 2150.

The control section 2100 outputs the located position QB2 to the displaydevice 2034 (see FIG. 15) based on the located position output program2130.

The terminal 2020 is configured as described above.

The terminal 2020 can calculate the correlation values of at least threefirst sampling phases CS1 in chip units.

The terminal 2020 can determine the first phase CPB0.

The terminal 2020 can calculate the first positioning phase CPB3.

When the signal strength is −155 dBm or more, the terminal 2020 cancalculate the located position QB1 using the first positioning phasesCPB3 corresponding to three or more GPS satellites 12 a and the like.

As described above, the correlation values of the phases EARLY and LATEmay become equal at a plurality of positions in a weak electric field.On the other hand, only one first phase CPB0 corresponds to the maximumcorrelation value.

Therefore, the true phase exists in the range of 1/32nd of a chip withrespect to the first phase CPB0.

Since the graph of the correlation value P forms an approximateisosceles triangle near the first phase CPB0 even in a weak electricfield, the first positioning phase CPB3 which is the phase correspondingto the vertex of the isosceles triangle can be calculated from threesampling phases including the first phase CPB0 and the correspondingcorrelation values P. The first positioning phase CPB3 is closer to thetrue phase than the first phase CPB0.

This enables the phase of the received positioning base code to beaccurately estimated even in a weak electric field in which the signalstrength is extremely low.

When the signal strength is less than −155 dBm, the terminal 2020 canperform the correlation process of the C/A code replica and the receivedC/A code in units of the second sampling phases CS2, and calculate thecorrelation value P.

The terminal 2020 can determine the second phase CPB02.

The terminal 2020 can calculate the second positioning phase CPB4.

Therefore, the second positioning phase CP4 is closer to the true phasethan the first positioning phase CPB3.

This enables the phase of the received C/A code to be accuratelyestimated even in a weak electric field in which the signal strength isextremely low.

The configuration of the terminal 2020 according to the secondembodiment has been described above. An operation example of theterminal 2020 is described below mainly using FIG. 25.

FIG. 25 is a schematic flowchart showing an operation example of theterminal 2020.

The terminal 2020 calculates the estimated frequency P (see FIG. 17) ofeach of the GPS satellites 12 a and the like from the ephemeris 152 band the initial position QB0 (see FIG. 17) (step S1 in FIG. 25).

The terminal 2020 performs the multiple division search (step S2). Thestep S2 exemplifies a first correlation value calculation step.

The terminal 2020 determines the first phase CPB0 (see FIG. 17)corresponding to the maximum correlation value Pmax (step S3). The stepS3 exemplifies a first phase determination step.

The terminal 2020 calculates the first positioning phase CPB3 based onthe first phase CPB0 and the phases CPB1 and CPB2 which advance or aredelayed from the first phase CPB0 (step S4). The step S4 exemplifies afirst positioning phase calculation step.

The terminal 2020 determines whether or not the signal strength is −155dBm or more (step S5).

When the terminal 2020 has determined that the signal strength is −155dBm or more in the step, S5, the terminal 2020 tracks under the firsttracking condition, and calculates the first positioning phase CPB3(step S6).

The terminal 2020 locates the present position using the firstpositioning phase CPB3, and calculates the located position QB1 (stepS7). The step S7 exemplifies a located position calculation step.

The terminal 2020 outputs the located position QB1 (step S8).

When the terminal 2020 has determined that the signal strength is not−155 dBm or more in the step S5, the terminal 2020 tracks under thesecond tracking condition, and calculates the second positioning phaseCPB4 (step S6A).

The terminal 2020 locates the present position using the secondpositioning phase CPB4, and calculates the located position QB2 (stepS7A).

The terminal 2020 outputs the located position QB2 (step S8A).

The above steps enable the phase of the received C/A code to beaccurately estimated even in a weak electric field in which the signalstrength is extremely low to a further extent.

Note that the invention is not limited to the above embodiments. Notealso that the above embodiments may be configured in combination.

Although only some embodiments of the invention have been describedabove in detail, those skilled in the art would readily appreciate thatmany modifications are possible in the embodiments without materiallydeparting from the novel teachings and advantages of the invention.Accordingly, such modifications are intended to be included within thescope of the invention.

1. A positioning device comprising: a peak frequency determinationsection which determines a peak frequency which is a reception frequencycorresponding to a maximum correlation value of a specific positioningbase code replica and a positioning base code carried on a radio wavefrom a specific transmission source; a reference frequency calculationsection which calculates a low frequency which is a frequency lower thanthe peak frequency and a high frequency which is a frequency higher thanthe peak frequency; a reference correlation value calculation sectionwhich calculates the correlation value corresponding to the lowfrequency and the correlation value corresponding to the high frequency;a corrected peak frequency calculation section which calculates acorrected peak frequency based on the correlation value corresponding tothe peak frequency, the peak frequency, the correlation valuecorresponding to the low frequency, the low frequency, the correlationvalue corresponding to the high frequency, and the high frequency; and aradio wave reception section which receives the radio wave using thecorrected peak frequency.
 2. The positioning device as defined in claim1, comprising: a reception frequency control section which controls thereception frequency so that a coherent value of the positioning basecode replica and the positioning base code is maximized.
 3. Thepositioning device as defined in claim 2, wherein the corrected peakfrequency calculation section and the reception frequency controlsection operate in parallel.
 4. The positioning device as defined inclaim 1, wherein the transmission source is a positioning satellite. 5.A positioning device comprising: a first correlation value calculationsection which performs a correlation process of a specific positioningbase code replica and a positioning base code and calculates acorrelation value in units of first sampling phases which are phases atintervals of a first divided phase width which is a phase width obtainedby equally dividing a phase range specified by a base unit of apositioning base code formed of a plurality of base units from atransmission source into at least three sections; a first phasedetermination section which determines a first phase which is a samplingphase corresponding to the maximum correlation value; a firstpositioning phase calculation section which calculates a firstpositioning phase used for positioning based on three consecutive firstsampling phases including the first phase and the correlation valuescorresponding to the three consecutive first sampling phases includingthe first phase; and a first located position calculation section whichcalculates a present located position based on the first positioningphases corresponding to three or more of the transmission sources. 6.The positioning device as defined in claim 5, comprising: a receptionstrength range determination section which determines whether or not astrength of a radio wave which carries the positioning base code iswithin a predetermined reception strength range; a second correlationvalue calculation section which performs the correlation process of thepositioning base code replica and the positioning base code andcalculates the correlation value in units of second sampling phaseswhich are phases at intervals of a second divided phase width obtainedby equally dividing a phase range specified by the base unit by thesecond divided phase width which is smaller than the first divided phasewidth based on the determination results of the reception strength rangedetermination section; a second phase determination section whichdetermines a second phase which is a phase of the positioning base codereplica corresponding to the maximum correlation value; a secondpositioning phase calculation section which calculates a secondpositioning phase used for positioning based on three consecutivesampling phases including the second phase and the correlation valuescorresponding to the three consecutive sampling phases including thesecond phase; and a second located position calculation section whichcalculates the present located position based on the second positioningphases corresponding to three or more of the transmission sources. 7.The positioning device as defined in claim 5, wherein the transmissionsource is a positioning satellite; wherein the positioning base code isa clear and acquisition or coarse and access (C/A) code; and wherein thebase unit is a chip forming the C/A code.
 8. A positioning controlmethod comprising: a peak frequency determination step of determining apeak frequency which is a reception frequency corresponding to a maximumcorrelation value of a specific positioning base code replica and apositioning base code carried on a radio wave from a specifictransmission source; a reference frequency calculation step ofcalculating a low frequency which is a frequency lower than the peakfrequency and a high frequency which is a frequency higher than the peakfrequency; a reference correlation value calculation step of calculatingthe correlation value corresponding to the low frequency and thecorrelation value corresponding to the high frequency; a corrected peakfrequency calculation step of calculating a corrected peak frequencybased on the correlation value corresponding to the peak frequency, thepeak frequency, the correlation value corresponding to the lowfrequency, the low frequency, the correlation value corresponding to thehigh frequency, and the high frequency; and a radio wave reception stepof receiving the radio wave using the corrected peak frequency.
 9. Apositioning control method comprising: a first correlation valuecalculation step of performing a correlation process of a specificpositioning base code replica and a positioning base code andcalculating a correlation value in units of first sampling phases whichare phases at intervals of a first divided phase width which is a phasewidth obtained by equally dividing a phase range specified by a baseunit of a positioning base code formed of a plurality of base units froma transmission source into at least three sections; a first phasedetermination step of determining a first phase which is a samplingphase corresponding to the maximum correlation value; a firstpositioning phase calculation step of calculating a first positioningphase used for positioning based on three consecutive first samplingphases including the first phase and the correlation valuescorresponding to the three consecutive first sampling phases includingthe first phase; and a first located position calculation step ofcalculating a present located position based on the first positioningphases corresponding to three or more of the transmission sources.
 10. Apositioning control program causing a computer to execute: a peakfrequency determination step of determining a peak frequency which is areception frequency corresponding to a maximum correlation value of aspecific positioning base code replica and a positioning base codecarried on a radio wave from a specific transmission source; a referencefrequency calculation step of calculating a low frequency which is afrequency lower than the peak frequency and a high frequency which is afrequency higher than the peak frequency; a reference correlation valuecalculation step of calculating the correlation value corresponding tothe low frequency and the correlation value corresponding to the highfrequency; a corrected peak frequency calculation step of calculating acorrected peak frequency based on the correlation value corresponding tothe peak frequency, the peak frequency, the correlation valuecorresponding to the low frequency, the low frequency, the correlationvalue corresponding to the high frequency, and the high frequency; and aradio wave reception step of receiving the radio wave using thecorrected peak frequency.
 11. A positioning control program causing acomputer to execute: a first correlation value calculation step ofperforming a correlation process of a specific positioning base codereplica and a positioning base code and calculating a correlation valuein units of first sampling phases which are phases at intervals of afirst divided phase width which is a phase width obtained by equallydividing a phase range specified by a base unit of a positioning basecode formed of a plurality of base units from a transmission source intoat least three sections; a first phase determination step of determininga first phase which is a sampling phase corresponding to the maximumcorrelation value; a first positioning phase calculation step ofcalculating a first positioning phase used for positioning based onthree consecutive first sampling phases including the first phase andthe correlation values corresponding to the three consecutive firstsampling phases including the first phase; and a first located positioncalculation step of calculating a present located position based on thefirst positioning phases corresponding to three or more of thetransmission sources.
 12. A computer-readable recording medium havingrecorded thereon a positioning control program which causes a computerto execute: a peak frequency determination step of determining a peakfrequency which is a reception frequency corresponding to a maximumcorrelation value of a specific positioning base code replica and apositioning base code carried on a radio wave from a specifictransmission source; a reference frequency calculation step ofcalculating a low frequency which is a frequency lower than the peakfrequency and a high frequency which is a frequency higher than the peakfrequency; a reference correlation value calculation step of calculatingthe correlation value corresponding to the low frequency and thecorrelation value corresponding to the high frequency; a corrected peakfrequency calculation step of calculating a corrected peak frequencybased on the correlation value corresponding to the peak frequency, thepeak frequency, the correlation value corresponding to the lowfrequency, the low frequency, the correlation value corresponding to thehigh frequency, and the high frequency; and a radio wave reception stepof receiving the radio wave using the corrected peak frequency.
 13. Acomputer-readable recording medium having recorded thereon a positioningcontrol program which causes a computer to execute: a first correlationvalue calculation step of performing a correlation process of a specificpositioning base code replica and a positioning base code andcalculating a correlation value in units of first sampling phases whichare phases at intervals of a first divided phase width which is a phasewidth obtained by equally dividing a phase range specified by a base:unit of a positioning base code formed of a plurality of base units froma transmission source into at least three sections; a first phasedetermination step of determining a first phase which is a samplingphase corresponding to the maximum correlation value; a firstpositioning phase calculation step of calculating a first positioningphase used for positioning based on three consecutive first samplingphases including the first phase and the correlation valuescorresponding to the three consecutive first sampling phases includingthe first phase; and a first located position calculation step ofcalculating a present located position based on the first positioningphases corresponding to three or more of the transmission sources.